Free Space Path Loss (FSPL) is the fundamental concept that describes the reduction in signal strength, or attenuation, of an electromagnetic wave as it propagates through a line-of-sight path in free space. This loss is not due to absorption by a medium but rather the geometric spreading of the wave's energy over a larger area as it travels away from the transmitter. According to the inverse-square law, the power density of the signal decreases proportionally to the square of the distance from the source. Therefore, as an RF signal gets farther from the transmitter, it becomes weaker.

B. RF latency: This refers to the time delay a signal experiences while propagating from the transmitter to the receiver, not the loss of signal strength.

C. Beamwidth: This is an antenna characteristic that defines the angle of the main lobe of its radiation pattern, describing the signal's directivity, not its attenuation over distance.

D. Diffraction: This is a propagation phenomenon where RF waves bend around the edges of an obstacle, which is different from the general signal weakening in open space.

1. Rappaport, T. S. (2002). Wireless Communications: Principles and Practice (2nd ed.). Prentice Hall. In Chapter 4, "Mobile Radio Propagation: Large-Scale Path Loss," Section 4.2, "Free Space Propagation Model," it is stated: "The free space power received by a receiver antenna... is directly proportional to the square of the wavelength and inversely proportional to the square of the T-R separation distance." (p. 101). This inverse relationship with the square of the distance is the definition of FSPL.

2. MIT OpenCourseWare. (2012). 6.02 Introduction to EECS II: Digital Communication Systems, Fall 2012. Lecture 15 Notes, "Wireless Communication." Section 15.2, "Path Loss," explains that received power (PR) is proportional to 1/d², where 'd' is the distance. This section explicitly defines path loss as the signal attenuation due to distance.

3. Ling, S. J., Sanny, J., & Moebs, W. (2016). University Physics Volume 1. OpenStax. In Chapter 16, "Waves," Section 16.8, "Energy and Power of a Wave," the text explains that for a spherical wave, "The intensity is therefore proportional to 1/r²," where 'r' is the distance from the source. This inverse-square law is the physical principle underlying Free Space Path Loss.